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Flow
curve
In order to determine the general flow behaviour
of a sample the viscosity is measured as a function of the shear
rate in a rotational rheometer. For the presentation of the
data either the viscosity or the shear stress is plotted against
the shear rate. The thus obtained graph is called flow curve.
In many cases the flow curve is the first and usually also the
most important rheological measurement. It shows the flow behaviour
for low shear rates (slow motions) as well as high shear rates
(fast motion). Low shear rate appear e.g. at the levelling or
sagging of paints, sedimentation or slow flowing. High shear
rates (> 1000 1/s) occur at the majority of technical processing
like coating, spraying or flowing though pipes.
By means of the flow curve it is possible to read the viscosity
for the shear rates of interest. If someone is interested in
the flow behaviour during the levelling the viscosity at shear
rates below 1 1/s are important. For the behaviour "on
the machine" the data at high shear rate are crucial.
For a better visualisation the following flow curve shows typical
shear rates for some practical applications
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Typical
shear rate for some technical applications.
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Basically
someone distinguish between ideal viscous, shear-thinning and
shear-thickening flow behaviour:
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Flow curves: (A) Ideal viscous; (B) shear-thinning, (C) shear-thickening
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Viscosity curves: (A) Ideal viscous; (B) shear-thinning, (C)
shear-thickening
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A)
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For
the ideal viscous flow the viscosity is independent on the
shear rate. This behaviour is common for homogenous low molecular
weight fluids like lubricating oil. For such samples it is
sufficient to indicate one value for the viscosity at a certain
temperature. The viscosities of ideal viscous samples are
normally determined by means of capillary of rolling ball
viscometers. The accuracy of these methods lies in the order
of one tenth of a percent and are therefore superior to rotational
rheometers. |
B)
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For
most of the inhomogeneous fluids, for polymer solutions and
for the majority of paste like and creamy products, the viscosity
decreases with increasing shear rate. This behaviour is called
shear-thinning. Most of the products of the common live (e.g.
paint, creams, sauces, glues or tooth paste) show shear-thinning
behaviour. |
C)
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Shear-thickening
flow behaviour shows up normally for highly filled dispersions
at high shear rates. One well-known example is starch in water.
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For samples that are not ideal viscous it is not sufficient
to indicate only the viscosity. It is always necessary to specify
also the shear rate that was used for the measurement.
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